1. Field of the Invention
This invention relates to an information recording apparatus for a magneto-optic recording information medium (magneto-optic disk), and more particularly, to the same for magneto-optic recording information medium having a so-called light modulation overwriting function which enables writing of new information directly over old recorded information.
2. Description of Related Art
A magneto-optic recording information medium, namely, a magneto-optic disk having a light modulation overwriting function, and a magneto-optic recording apparatus were proposed in the Japanese Patent Application Laid-Open No. 1-119244 (1989).
The proposed magneto-optic recording information medium included a first magnetic layer having vertical magnetic anisotropy and a second magnetic layer, which also has vertical magnetic anisotropy, laid on the first magnetic layer and is bonded to it with exchange force. The medium is characterized in that the second magnetic layer
(a) does not cause flux reversal and keeps its direction of magnetization constant at recording and producing, PA1 (b) meets the requirement of Tc.sub.1 &lt;Tc.sub.2 PA1 (c) meets the requirements of EQU Hc.sub.1 &gt;Hw.sub.1 +Hb, Hc.sub.2 &gt;Hw.sub.2 +Hb PA1 at room temperatures, PA1 (i) Because the laser beam 20 is projected from the side of the first magnetic layer 13, the maximum reachable temperature of the first layer 13 is higher than that of the second layer 14 and thus the heat radiation rate of the first layer 13 is higher than that of the second layer 14. PA1 (ii) The first magnetic layer 13 is adjacent to the substrate 2 and radiates heat through the substrate 2. PA1 (iii) The thickness of the first magnetic layer 13 is very thin, therefore heat radiation is great. PA1 first magnetic layer 13: Tb.sub.23 Fe.sub.72 Co.sub.5 (thickness 500 .ANG.) and PA1 second magnetic layer 14: Gd.sub.14 Tb.sub.14 Co.sub.72 (thickness 1500 .ANG.) on a glass substrate 2 by the sputtering method, for example. The magnetic layers are bonded together by exchange force. PA1 first magnetic layer: Tb.sub.23 Fe.sub.67 Co.sub.10 (thickness 500 .ANG.) and PA1 second magnetic layer: Gd.sub.12 Tb.sub.12 Co.sub.76 (thickness 1500 .ANG.)
where PA2 Tc.sub.1 is the Curie temperature of the first magnetic layer, PA2 Tc.sub.2 is the Curie temperature of the second magnetic layer and PA2 where PA2 Hc.sub.1 is the coercive force of the first magnetic layer PA2 Hc.sub.2 is the coercive force of the second magnetic layer PA2 Hw.sub.1 is the shift quantity of inversion magnetic field due to the exchange force of the first magnetic layer PA2 Hw.sub.2 is the shift quantity of inversion magnetic field due to exchange force of the second magnetic layer PA2 Hb : applied magnetic field at recording (Hb&gt;0).
"The proposed magneto-optic recording apparatus included a magneto-optic recording information medium having at least two magnetic layers with vertical magnetic anisotropy, one layer of which keeps its direction of magnetization constant and does not cause flux reversal at recording and reproducing, a beam emitting element which projects a beam on the magneto-optic recording information medium to record or reproduce information, and a magnetic field generator which generates a magnetic field to be applied to a portion of the magneto-optic recording information medium projected by the beam emitting element while keeping the direction of magnetic field constant.
Now, referring to an drawings, the explanation of this medium and apparatus will be given in more detail as follows.
FIG. 1(a) is a schematic diagram showing an outline of a magneto-optic recording information medium and the essential part of a magneto-optic recording apparatus which records information on the medium as have been proposed in the Japanese Patent Application Laid-Open No. 1-119244 (1989) mentioned above. FIG. 1(b) is a partial section taken along a circumference of the magneto-optic recording information medium and also includes a graph showing the variation of laser beam power for information recording on the magneto-optic recording information medium.
In FIG. 1(a) and (b), numeral 11 denotes a magneto-optic recording information medium, 20, a laser beam from a laser beam emitting element which projects the beam onto the magneto-optic recording information medium 11 to record or reproduce information, and 16, a beam spot which is generated by condensing the laser beam 20 through an objective lens 5 to be projected on the magneto-optic recording information medium 11.
Numeral 18 denotes a magnetic field generator which generates a magnetic field having a constant direction and is applied to a laser beam projected portion on the magneto-optic recording information medium 11.
Numeral 2 denotes a substrate made of glass or plastics.
Numeral 13 denotes a first magnetic layer, which is laminated on the substrate 2 and has vertical magnetic anisotropy.
Numeral 14 denotes a second magnetic layer, which is laminated on the first magnetic layer 13 and has vertical magnetic anisotropy. The second layer 14 is bonded to the first magnetic layer 13 with exchange force and does not cause flux reversal at recording or reproducing, thus keeping the direction of magnetization constant.
Numeral 7 denotes an area with binary data "1" indicating that the direction of magnetization of the first magnetic layer 13 is directed upward in FIG. 1(b).
The first magnetic layer 13 and the second magnetic layer 14 are composed of a rare earth metal-transition metal alloy and have relations Tc.sub.1 &lt;Tc.sub.2 (where Tc.sub.1 and Tc.sub.2 are the Curie temperatures of the first and the second magnetic layers 13 and 14 respectively) and Hc.sub.1 &gt;Hw.sub.1 +Hb, Hc.sub.2 &gt;Hw.sub.2 +Hb (where Hc.sub.1 and Hc.sub.2 are the coercive forces of the first and the second magnetic layers 13 and 14 at a room temperature, Hw.sub.1 and Hw.sub.2 are the exchange bonding force of the first and the second magnetic layers 13 and 14 at a room temperature, and Hb is a magnetic field generated by the magnetic field generator.
In order to perform the so-called light modulation direct overwriting, it is necessary to control the intensity of the laser beam 20 from the laser beam emitting element to three levels: high, intermediate and low. At the high and the intermediate levels of the laser pulse beam, in the case when a magnetic layer other than one which does not cause flux reversal is at the high level, either one of a pit (mark) with upward direction of magnetization or a pit with downward direction of magnetization is formed and when at the intermediate level, the other pit is formed. Thus reading of information can be performed with the low level laser beam.
Next, the operation of this apparatus will be described.
The magneto-optic recording information medium 11 is rotated in the direction of an arrow a in the drawing. This medium 11 has two magnetic layers 13 and 14 as described above and is formed with a substrate 2, the first magnetic layer 13 and the second magnetic layer 14 in order from the side of laser projection.
Now, the first magnetic layer 13 is a reading layer as well as a recording layer for holding magnetization orientation indicative of information "0" or "1" and the second magnetic layer 14 is provided to effect overwriting. This second magnetic layer 14 is called an initialization layer and has both the functions of the conventional auxiliary layer and the initialization magnet.
Characteristics of the first magnetic layer 13 and the second magnetic layer 14 are as follows:
Giving notations Tc.sub.1 and Tc.sub.2 to each Curie temperature of both the layers, then EQU Tc.sub.1 &lt;Tc.sub.2.
Further, giving notations Hc.sub.1 and Hc.sub.2 to each coercive force of both the layers, and notations Hwi (i=1, 2) to each exchange bonding force of both the layer, then EQU Hc.sub.1 &gt;Hw.sub.1 +Hb (1) EQU Hc.sub.2 &gt;Hw.sub.2 +Hb (2).
Inequality (1) holds within the range of room temperatures to a certain temperature T.sub.0 lower than Tc.sub.1. That is, in the range from temperature to temperature T.sub.0, the coercive force Hc.sub.1 of the first magnetic layer 13 is greater than a sum of the effect of the exchange bonding force Hw.sub.1 and the applied magnetic field Hb at recording which is generated by the magnetic field generator 18 and is not affected by the direction of magnetization of the second magnetic layer 14 and is able to hold the direction of magnetization indicative of recorded information.
Inequality (2) holds within the whole range of operating conditions. That is, in the whole range of operating conditions, the coercive force Hc.sub.2 of the second magnetic layer 14 is greater than a sum of the effect of the exchange bonding force Hw.sub.2 and the applied magnetic field Hb at recording which is generated by the magnetic field generator 18. Therefore, once the second magnetic layer 14 is initialized upward as shown in FIG. 1(b), the direction of magnetization is not reversed and the upward direction of magnetization can be mentioned.
Explanation of reproducing information recorded on the first magnetic layer 13 will now be described.
As shown in FIG. 1(b), the first magnetic layer is magnetized upward or downward in a thickness corresponding to a binary code "1" or "0". When reproducing information, the beam spot 16 is projected on this first magnetic layer 13 and the direction of magnetization of this projected area of the first magnetic layer 13 is converted into optical information with the well-known optical Kerr effect and thus information recorded in the magneto-optic recording information medium 11 is detected.
In this case, the intensity of the laser projected on the magneto-optic recording information medium 11 is one at point A in a graph of FIG. 3 described later. In the first and the second magnetic layers 13 and 14, the maximum temperature on the beam spot 16 projected by the light beam of this intensity does not reach the respective Curie temperature Tc.sub.1, Tc.sub.2 of both the layers. Therefore, magnetizing information is not eliminated by beam projection of the beam spot 16.
A relation between temperatures and the inversion magnetic fields of the first magnetic layer 13 is shown in a graph of FIG. 2 and a relation between the intensity of the laser beams on the magneto-optic recording information medium 11 and temperatures of the magnetic layer in the laser spot is shown in the graph of FIG. 3. An inversion magnetic field is the minimum field required to reverse a direction of magnetization and is expressed by EQU Hc.sub.1 -Hw.sub.1.
When a laser intensity (power) R.sub.1 is applied as shown in FIG. 1(b), a relation between inversion magnetic fields and temperatures of the first magnetic layer is shown in a solid line in FIG. 2 and when a laser intensity (power) R.sub.0 is applied, the relation is shown in a broken line.
The operation of recording information "0", that is downward of causing magnetization to be given to the first magnetic layer 13 will now be explained.
When the laser beam 20 with intensity R.sub.1 is projected, the temperature of the first magnetic layer 13 in the beam spot 16 rises to Tr.sub.1 in FIG 2. Then, when the disk is rotated and the laser beam 20 is not projected on the beam spot 16, the temperature of the first magnetic layer 13 falls. As can be seen from the solid line in FIG. 2, the following inequality is valid within the range of room temperatures to Tc.sub.1 : EQU .vertline.Hb.vertline.&gt;Hw.sub.1 -Hc.sub.1
Therefore, the direction of magnetization of the first magnetic layer 13 is the direction of the magnetic field generated by the magnetic field generator 18, that is, the direction of a biasing magnetic field Hb, namely downward direction.
The operation of recording information "1", that is, of causing an upward direction of magnetization to be given to the first magnetic layer 13 will now be explained.
When the laser beam with its intensity R.sub.0 is projected, the temperature of the first magnetic layer 13 in the beam spot 16 rises to Tr.sub.0 in FIG. 2. Then, when the disk is rotated and the laser beam 20 is not projected on the beam spot 16, the temperature of the first magnetic layer 13 falls. As can be seen from the broken line in FIG. 2, the following inequality is valid in the vicinity of the temperature Tp: EQU .vertline.Hb.vertline.&lt;Hw.sub.1 -Hc.sub.1.
Therefore, the direction of magnetization of the first magnetic layer 13 is the direction in which the exchange force acts, that is, the direction of magnetization of the second magnetic layer 14, namely the upward direction.
Then, when overwriting is performed by the above operation, the laser beam is intensity-modulated to become R.sub.1 or R.sub.0, that is, the intensity at point C or B in FIG. 3 according to the binary code "0" or "1" of information, thus the overwriting can be effected on old data in real time without requiring magnets for initializing.
The laser intensity at point A in FIG. 3 is the intensity used for reading information as mentioned above. Using this intensity at point A, the maximum temperatures of the first and the second magnetic layers 13 and 14 in the beam spot 16 do not reach respective Curie temperature Tc.sub.1 and Tc.sub.2 of both the layers. Therefore, the direction of magnetization, namely, recorded information, is not eliminated by beam projection on the beam spot 16.
Now, the reason why the curve of temperatures of the inversion magnetic fields in the first magnetic layer 13 separates into the broken line curve and the solid one according to the laser intensities R.sub.0 or R.sub.1 as shown in FIG. 2 will be explained.
Both the magnetic layers 13 and 14 cause the temperature rise due to laser projection, as to heat radiation rate, the first layer 13 has a higher one than that of the second layer 14. The reason is as follows.
Thus, the heat radiation rate of the first magnetic layer 13 is higher than that of the second magnetic layer 14. Due to the projection of the laser beam 20 with its intensity R.sub.0, the temperature of the first magnetic layer 13 rises to Tr.sub.0 in FIG. 2 and after that drops to around Tp in FIG. 2. At this time, the temperature of the second magnetic layer 14 is denoted T.sub.2 r.sub.0. Due the projection of the laser beam 20 with its intensity R.sub.1, the temperature of the first magnetic layer 13 rises to Tr.sub.1 in FIG. 2 and thereafter the temperature of the first magnetic layer 13 drops to around Tp in FIG. 2. At this time the temperature of the second magnetic layer 14 is denoted T.sub.2 r.sub.1, then due to difference between the heat radiation rates mentioned above, EQU T.sub.2 r.sub.0 &lt;T.sub.2 r.sub.1
results.
That is, when the laser beam 20 with its higher intensity R.sub.1 is projected, the temperature of the second magnetic layer 14 becomes higher when the temperature of the layer 13 is about Tp. Considering that the exchange bonding force has a tendency to decrease as the temperature of the magnetic layer becomes high, the exchange bonding force becomes small when the laser beam 20 with its higher intensity R.sub.1 is projected. Therefore, the difference in FIG. 2 arises between the solid line and the broken line curves of the temperature varying of inversion magnetic fields of the first magnetic layer 13. This causes magnetization hysteresis in relation to temperature and enables overwriting.